1. Art Field
The present invention relates generally to an organic EL (electroluminescent) device, and more specifically to an inorganic/organic junction structure used for a device comprising an organic compound thin film which emits light at an applied electric field.
2. Background Art
An organic EL device is now under development and investigation so that it can be used for display purposes, because it can be formed over a large area. In general, an organic EL device is basically built up of an ITO or other transparent electrode formed on a glass substrate, an organic amine base hole transporting layer laminated on the transparent electrode, an organic light emitting layer formed of a material having electronic conductivity and giving out strong light emission, for instance, an Alq.sup.3 material, and an electrode provided on the organic light emitting layer and formed of a material having a low work function, for instance, an MgAg material.
As reported so far in the art, the device has a structure wherein one or plural organic compound layers are interleaved between a hole injecting electrode and an electron injecting electrode. The organic compound layer has a double- or triple-layer structure.
Examples of the double-layer structure are a structure wherein a hole transporting layer and a light emitting layer are formed between the hole injecting electrode and the electron injecting electrode, and a structure wherein a light emitting layer and an electron transporting layer are formed between the hole injecting electrode and the electron injecting electrode. In an exemplary triple-layer structure, a hole transporting layer, a light emitting layer and an electron transporting layer are provided between the hole injecting electrode and the electron injecting electrode. A single-layer structure wherein a single layer has all functions, too, is reported in conjunction with a polymer or mixture system.
Typical structures of the organic EL device are shown in FIGS. 3 and 4.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15, each made of an organic compound, are formed between a hole injecting electrode 12 provided on a substrate 11 and an electron injecting electrode 13. In this case, the light emitting layer 15 also functions as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15 and an electron transporting layer 16, each made of an organic compound, are formed between a hole injecting electrode 12 provided on a substrate 11 and an electron injecting electrode 13.
A problem common to these organic EL devices is reliability. In principle, an organic EL device comprises a hole injecting electrode and an electron injecting electrode and requires an organic layer for efficient injection and transportation of holes and electrons from between these electrodes. However, these materials are sensitive to damages during device fabrication, and offer a problem in conjunction with an affinity for electrodes. Another problem is that the deterioration of an organic thin film is much severer than that of an LED or LD.
An electroluminescent (EL) device emits light under the influence of an electric field. The action of a semiconductor layer forming such an EL manifests itself through radiative combination of electron-hole pairs injected from a pair of electrodes into the semiconductor. One example of this is a light emitting diode based on a GaP semiconductor or other similar group III-V semiconductor. Although these devices are utilized effectively and in wide fields, yet application thereof to large-area displays is not only difficult but also uneconomical because their size is very minute. Some replacements applicable to large-area displays are known in the art. Of such inorganic semiconductors, ZnS is the most useful. However, one non-negligible practical problem with this system is that it is poor in reliability. In one mechanism to which ZnS relates, a kind of carrier is accelerated through the semiconductor in a strong electric field. This is believed to cause local excitation of the semiconductor, which decays upon radiative emission.
Of organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electro-luminescence.
Practical problems with these materials are that they do not only lack reliability as is the case of ZnS, but it is also difficult to bond organic layers formed thereof to a current-injecting electrode layer.
A problem with techniques of sublimating or otherwise treating organic materials is that the resultant layer is susceptible to re-crystallization due to its softness.
Techniques such as Langmuir-Blodgett evaporation of suitably modified aromatic compounds give rise to degradation of films, dilution of active substances, and production cost increases.
An electroluminescent device making use of anthracene is disclosed in U.S. Pat. No. 3,621,321. Disadvantages of this device are that much power is consumed in spite of low luminescence.
With a view to providing an improved device, U.S. Pat. No. 4,672,265 describes an electroluminescent device having a double-layer structure of light emitting layer.
However, substances proposed for the double-layer structure are organic materials having such disadvantages as mentioned above.
JP-A 10-92576 discloses an electroluminescent device comprising a thin yet intimate polymer film form of semiconductor layer comprising at least one conjugated polymer, a first contact layer coming into contact with a first surface of the semiconductor layer and a second contact layer coming into contact with a second surface of the semiconductor layer. The polymer film of the semiconductor layer has external charge carriers at a concentration so sufficiently low that when an electric field is applied between the first and second contact layers by way of the semiconductor layer while the second contact layer is positive with respect to the first contact layer, the charge carriers can be injected in the semiconductor layer to emit light from the semiconductor layer.
The conjugated polymer per se, too, is known in the art, and a practical application of the polymer to an optical modulator is put forward in, for instance, European Patent Application No. 0294061. In this case, polyacetylene is used as an active layer in a modulation structure between the first and second electrodes. It is then required that between one electrode and the active layer there be provided an insulating layer so as to form a spatial charge area in the active layer contributing to optical modulation effect. However, the presence of the spatial charge layer makes it impossible to achieve an electron/hole pair which decays to emit light. In other words, such a structure cannot possibly show electroluminescence. In any case, the fact that electroluminescence is obtained in European Patent Application No. 0294061 is quite undesired because the optical modulation effect is destroyed thereby.
To provide a solution to such problems, methods of taking advantage of merits of both an organic material and an inorganic semiconductor material have been envisaged. That is, an organic/inorganic semiconductor junction structure wherein an organic hole transporting layer is substituted by an inorganic p-type semiconductor has been contemplated. Such contemplation has been investigated in Japanese Patent No. 2636341, and JP-A's 2-139893, 2-207488 and 6-119973. However, it is still difficult to obtain an organic EL device superior to prior art crganic ELs in terms of emission performance and basic device reliability.